The "Second Great Fire of London" refers to one of the most destructive air raids of the Blitz. An American correspondent in a cable to his office said, "The second Great Fire of London has begun".[2] The Luftwaffe raid caused fires over a greater area than the Great Fire of London in 1666.[3]

On the night of 29–30 December 1940, approximately 100,000 bombs fell on the city.[3] The Germans dispatched 136 bombers to the city. Fewer incendiaries were dropped than in the raids of 15 November or 8 December. However, the raid was focused on a part of the city that contained many non-residential buildings, such as churches, offices, and warehouses. Many of these were locked and were not covered by the Fire Watchers Order of September 1940,[4] which applied to places of work with at least 30 employees, warehouses with an area of 50,000 cubic feet (1,400 m3) and sawmills or timber yards with more than 50,000 cubic feet of timber.[5]

Incendiary bombs were the main armament that night. Moderately small, at 12 by 3 inches (30 cm × 8 cm) — each bomber was equipped with approximately 180 of them — the bombs carried magnesium and started fires. The raid saw 1,500 fires begin in the city.[3]

Firefighters' efforts were hampered by a water shortage. The main water-main in the City was bombed. Units trying to get water from other hydrants caused that the water pressure to drop. Efforts to draw water from the River Thames were hampered by the low tide.[3] The fire brigade's difficulties were further exacerbated by wind, combined with the concentrated area of the attack.[4]

Twelve of those killed in the raid were firefighters, while 250 were injured.[3]

Artist Leonard Rosoman was serving with the Auxiliary Fire Service on the night of the raid. While fighting a fire in Shoe Lane, Rosoman was relieved of his hose. Moments later a wall collapsed, burying the two firefighters working where Rosoman had just been. The moment haunted Rosoman for the rest of his life. He immortalised the scene in his painting A House Collapsing on Two Firemen, Shoe Lane, London, EC4.William Sansom, a friend of Rosoman's who would go on to become a novelist and travel writer, survived the incident. The fireman who relieved Rosoman of his hose was killed.[6]

Of those firefighters lost, eight were killed when a wall collapsed on them as they worked at a fire near Fleet Street.[7] Sam Chauveau of the London Fire Brigade described the scene that faced the fire fighters: [2]

By the time we finished tackling the fires on the roof of the [Stock] Exchange, the sky, which was ebony black when we first got up there, was now changing to a yellowy orange colour. It looked like there was an enormous circle of fire, including St Paul's churchyard.

I focused at intervals as the great dome loomed up through the smoke... The glare of many fires and sweeping clouds of smoke kept hiding the shape. Then a wind sprang up. Suddenly, the shining cross, dome, and towers stood out like a symbol in the inferno. The scene was unbelievable. In that moment or two, I released my shutter.

The camera is believed to have been a Van Neck on a quarter plate glass negative. The Royal Photographic Society, Magazine Photographic Journal, remarked on the strength of the light: [9]

The light that was available for an instantaneous exposure is an indication of the fierceness and extent of the fire.

Volunteers serving as part of the St Paul’s Watch patrolled the iconic building.[8] The Watch was first put together during the First World War. Then it was meant to protect the building from German Zeppelin raids. With the outbreak of the Second World War, the group was reconstituted to guard against the anticipated air raids.[8]

On the night of 29 December, a little after 6.30 pm, an incendiary bomb broke through the cathedral's dome and wedged in its timbers.[7]

Famous American reporter Ed Murrow stated in a broadcast to the United States: [7]

The church that means most to London is gone. St Paul’s Cathedral is burning to the ground as I talk to you now.

The reports of the cathedral's demise were premature. The incendiary burnt through the wood and fell to the nave. There it was put out by members of the St Paul's Watch.[7]

Fellow American correspondent Ernie Pyle observed the raid from a balcony: [10]

Into the dark shadowed spaces below us, while we watched, whole batches of incendiary bombs fell. We saw two dozen go off in two seconds. They flashed terrifically, then quickly simmered down to pin points of dazzling white, burning ferociously…

The greatest of all the fires was directly in front of us. Flames seemed to whip hundreds of feet into the air. Pinkish-white smoke ballooned upward in a great cloud, and out of this cloud there gradually took shape—so faintly at first that we weren’t sure we saw correctly—the gigantic dome of St Paul’s Cathedral.
St Paul’s was surrounded by fire, but it came through. It stood there in its enormous proportions—growing slowly clearer and clearer, the way objects take shape at dawn. It was like a picture of some miraculous figure that appears before peace-hungry soldiers on a battlefield.

Publishing bore heavy losses in the raid. Ave Maria Lane and Paternoster Row, an area known since the 19th century as the center of the London publishing and book trade,[11][12][13][14] were badly hit, and the buildings and stock of 20 publishing houses were totally or partially destroyed. Stationers' Hall, neighbouring offices, the book wholesalers Simpkin Marshall, and several bookshops were lost. An estimated five million books were lost in the fires caused by tens of thousands of incendiary bombs.[15]